1. Field of the Invention
This invention relates to the stimulation of oil wells in subterranean formation by matrix acid stimulation or fracturing. More particularly, it relates to a method of employing a new viscoelastic surfactant acid composition to tolerate high iron ions.
2. Description of Related Art
Acidizing is a method in which an acidic fluid comes in contact with a subterranean formation, known as the matrix, which is penetrated by a wellbore. The acidizing fluid contacts and dissolves wellbore damage and/or part of the matrix.
The typical matrix acid stimulation is a multistage process which includes alternating stages. Initially, an emulsified acid (20 wt % HCl) is injected, which is followed by injecting a spacer (non-emulsified 20 wt % HCl) which typically includes certain additives, such as a corrosion inhibitor, an iron control agent, a water wetting surfactant and an H2S scavenger, as needed. Thereafter, a viscoelastic diverting agent (VDA)/agent is injected.
It is important that the injected acid reach the entire target zone to achieve maximum benefit. This is often difficult to achieve because of a natural tendency of the acid to react with the first reactive formation rock with it contacts, whether it is closest to the wellbore, or at its most porous portion, in matrix acidizing. Attempts to achieve complete contact of acid with an entire rock formation zone, which is referred to as zonal coverage, involves diversion of the acid flow from the region first contacted (high permeability) to new less permeable regions (more damaged) regions by injection of the above-referenced VDA.
One method of achieving zonal coverage which has met with a good deal of success in the last number of years is by placement of a gel or a foam, after treatment of a zone, which impedes fluid flow into the treated zone and diverts the acid into a new, still untreated, zone. Such viscoelastic surfactant-based acids are usually either amphoteric or cationic. Once the acid reacts with the carbonate formation and the pH increases above 2, and the concentrations of calcium and magnesium ions increases, these surfactants form long rod-shaped micelles. As a result, the viscosity of the solution increases significantly. This results in better acid diversion in matrix acid stimulation treatments.
Acid diversion is considered an imperative issue which contributes to the success of any acid stimulation treatment. A good diversion system results in improved zonal coverage resulting in increased hydrocarbon output from the stimulation treatment.
A problem that has been encountered when employing a viscoelastic diverting agent (VDA), such as a viscoelastic surfactant (VES), is its interaction with iron. (Paper SPE 117060, Nakhli et al. 2008). Interaction of iron ions arises from corrosion or formation water which tends to change the properties of the VDA, which renders it unsuitable for acid diversion.
During a matrix acidizing treatment, the acid can be contaminated with iron compounds at all stages. Rust present in storage tanks and mixing tanks can be dissolved by acid and produce a mixture of iron (II) and iron (III). Large quantities of iron ions in live acids can result when acid dissolves mill scale in new tubing or corrosion products. According to the literature, the content of iron at the wellhead can vary from 200 to 3500 mg/L and the returning acid iron concentration can vary from 9,000 to 100,000 mg/L. Also, formation brine may contain iron.
The interaction of iron with VDA/acid system will adversely affect the performance of VDA. The following adverse effects can be expected:                phase separation which will result in malfunction of the VDA which results in no diversion taking place, and        precipitation which will result in damaging well productivity or injectivity.        
In matrix acidizing treatments where a viscoelastic surfactant is not employed, very small quantities of chelating agents are added to prevent iron precipitation. In those instances in the prior art where a viscoelastic diverting agent, e.g. a surfactant, is employed in conjunction with acidizing, the chelating agents that are added are at a very low concentration. For example, Chen et al U.S. Pat. No. 7,320,952 discloses the addition of just 0.05%, by weight, of a chelating agent, based on actives, which is done solely to prevent iron precipitation. There is no disclosure of preventing interaction with the viscoelastic surfactant which is present. Chen et al. teaches the addition of iron chelating or sequestering agents to prevent iron precipitation once the acid is spent due to its reaction with formation rock or acid reactive minerals.